In order to study space plasma physics problems, it is convenient and often times necessary to carry out experiments in the laboratory. In that case, the plasma frequently must be of large dimensions and of uniform density. In addition, it is usually necessary to provide a magnetic field extending through the plasma. Furthermore, the plasma should be collisionless, free of instabilities, adjustable and reproducible over wide parameter ranges.
For these purposes, direct current discharge plasmas are well suited. Such plasmas utilize hot cathodes and grided anodes. Cathodes best suited for this purpose are those which are indirectly heated and oxide-coated. Such cathodes provide large emission currents on the order of 1 ampere per square centimeter (cm.sup.2), at relatively low temperatures such as about 800.degree. centigrade (.degree.C.).
Cathodes of this type are commercially available. However, they are generally restricted to diameters less than 10 cm. The construction of large cathodes having diameters of 50 cm or more encounters difficulties which are more severe than those encountered in the construction of smaller cathodes. For example, the thermal expansion due to temperature gradients or the use of different materials and the coating and activation processes have to be specifically treated in order to obtain a uniformly emissive planar surface having a useful lifetime in a high density plasma.
It is accordingly an object of the present invention to provide a large, oxide-coated, indirectly heated cathode for generating a large and high density plasma.
A further object of the present invention is to provide a large cathode of the type discussed, which may be maintained substantially flat or planar in spite of large variations of temperature.
Another object of the present invention is to provide a cathode of the type discussed where the generation of a magnetic field due to the currents flowing through the heater wires is minimized.
Still a further object of the present invention is to provide a large, flat cathode having a large heating capacity and which may be cooled by the flow of a fluid.
The large cathode structure of the present invention having a diameter of about 50 cm has been in use for some time. Some of the experiments which one of the applicants has performed with this large plasma have been described in a series of papers. See, for example, a paper by R. L. Stenzel entitled "Microwave Resonator Probe for Localized Density Measurements in Weakly Magnitized Plasmas," which appears in "Review of Scientific Instruments," Volume 47, May 1976, pages 603-607; another paper by the same author entitled "Whistler Wave Propagation in a Large Magnetoplasma," which appears in "The Physics of Fluids," Volume 19, June 1976, pages 857-864; and another paper by the same author entitled "Filimentation Instability of a Large Amplitude Whistler Wave," in "The Physics of Fluids," Volume 19, June 1976, pages 865-871. The generation of a large electron beam is described in the paper by R. L. Stenzel entitled "Unstable Whistler Wave Propagation Along the Resonance Cone in a Large Beam-Plasma System," which appeared in "Physical Review Letters", Volume 38, Number 8, Feb. 21, 1977, pages 394-397. While these papers refer to a 50 cm diameter oxide-coated cathode, the construction of this cathode has not been revealed in these papers.
It should be noted that such relatively large plasmas of the type discussed may, for example, be used for measurements in tests of a satellite. They may also be used for generating a large, dense beam by accelerating the resulting plasma. Plasmas of this type may be made with any of the noble gases such as helium, argon, krypton, xenon, neon, etc., or even with alkali and other metals such as uranium. Additionally, the cathode structure itself may be utilized for the production of large diameter electron beams.